Neuron, Vol. 24, 299±312, October, 1999, Copyright 1999 by Cell Press The Evolution and Physiology of Human Review Color Vision: Insights from Molecular Genetic Studies of Visual Pigments Jeremy Nathans* Present-day vertebrates vary enormously in the so- Department of Molecular Biology and Genetics phistication of their color vision, the density and spatial Department of Neuroscience and distribution of cone classes, and the number and ab- Department of Ophthalmology sorption maxima of their cone pigments (Figure 2; Howard Hughes Medical Institute Lythgoe, 1979; Jacobs, 1981, 1993; Yokoyama, 1997). Johns Hopkins University School of Medicine At one extreme, most mammals have only three pig- Baltimore, Maryland 21205 ments: the two ancestral cone pigments and rhodopsin. At the other evolutionary extreme, chickens possess six Nothing in biology makes sense except in light of evo- pigments: four cone pigments, one rhodopsin, and a lution. pineal visual pigment, pinopsin. As seen in the dendro- ÐT. Dobzhansky gram in Figure 2, the chicken green pigment was derived from a duplication within the rhodopsin branch. Color vision is the process by which an organism ex- In this evolutionary comparison, humans and their tracts information regarding the wavelength composi- closest primate relatives represent an intermediate level tion of a visual stimulus (Figure 1). In its simplest formÐ of complexity. Humans have four visual pigments: a exemplified by the wavelength-dependent phototactic single member of the ,500 nm family of cone pigments responses of halobacteriaÐcolor vision is based on the (the blue or short-wave pigment, with an absorption relative abundances of two isoforms of a sensory pig- maximum at z425 nm), two highly homologous mem- ment (Hoff et al., 1997). One isoform preferentially ab- bers of the .500 nm family (the green or middle-wave sorbs long wavelength light and mediates a photoat- pigment, and red or long-wave pigment, with absorption tractant response, and the second isoform preferentially maxima at z530 and z560 nm, respectively), and rho- absorbs short wavelength light and mediates a photore- dopsin. The presence of only a single gene encoding a pellant response. Light absorption photoconverts the .500 nm pigment in almost all New World primates, first isoform into the second, and the second into the and in all nonprimate mammals studied to date, places first. Thus, the steady-state ratio of pigment isoforms the red/green visual pigment gene duplication in the provides a measure of the spectral composition of the Old World primate lineage at z30±40 million years ago, ambient light. shortly after the geologic split between Africa and South The halobacterial system contains the two cardinal America (Jacobs, 1993). Current molecular genetic evi- elements of every color vision system: (1) two or more dence suggests that howler monkeys, the only known sensory pigments with different spectral sensitivities (al- species of New World primate with two .500 nm pig- though in most organisms these are distinct chromopro- ment genes, have acquired a gene duplication event teins rather than isoforms of a single chromoprotein), that is independent of the one within the Old World and (2) a mechanism for monitoring the relative number primate lineage (Jacobs et al., 1996; Hunt et al., 1998; of photons captured by the different pigments. In higher Kainz et al., 1998). The relatively recent acquisition of eukaryotes, this system has evolved so that, in general, trichromacy by Old World primates is reflected in the each pigment resides within a distinct class of photore- structure of the red and green pigment genes (Nathans ceptor cells, and therefore the ratios of photoexcitation et al., 1986a; Vollrath et al., 1988; Ibbotson et al., 1992). of the different pigments can be determined by as- These genes reside in a head-to-tail tandem array on sessing the ratios of activation of different photorecep- the X chromosome and show z98% DNA sequence tor cells. identity in the coding, intron, and 39 flanking sequences. Whether the Old World duplication involved initially iden- Evolution of Vertebrate Visual Pigments tical genes that subsequently diverged, or whether it Visual pigments are G protein±coupled receptors in arose from two X chromosomes that carried polymor- which a seven-transmembrane segment protein is cova- phic variants with different absorption spectra, is not lently linked to a chromophore, 11-cis retinal. Studies known. of the visual pigment complement and color vision ability of different vertebrates reveal an ancient and nearly uni- Spectral Tuning: Mechanisms versal color vision system in which one visual pigment and Sequence Determinants has an absorption maximum at ,500 nm and a second A long-standing challenge in vision research has been to visual pigment has an absorption maximum at .500 nm explain the mechanisms by which diverse visual pigment (Mollon, 1989; Yokoyama, 1997). Rhodopsin, a third and apoproteins regulate the wavelength of absorption of equally ancient pigment, has an absorption maximum the common 11-cis retinal chromophore, and to relate at z500 nm and plays little or no role in color vision. In these absorption spectra to the visual abilities and evo- general, the pigments mediating color vision reside in lutionary histories of each species. cone photoreceptors and are used only under bright The basic photochemical properties of 11-cis retinal light conditions, whereas rhodopsin resides in rod pho- are now well understood, and these constrain the strate- toreceptors and is used under dim light conditions. gies that a visual pigment apoprotein can use to effect an absorption shift (Ottolenghi and Sheves, 1989; Birge, * E-mail: [email protected]. 1990). In all visual pigments, 11-cis retinal is joined to Neuron 300 Figure 1. The Practical and Aesthetic Advantages of Color Vision The figure shows the same photograph of autumn foliage with and without color. Color vision permits distinctions between objects based on differences in the chromatic composition of reflected light. the apoprotein by a Schiff base linkage with a lysine in positively charged (Figure 3A). All vertebrate visual pig- the center of the seventh transmembrane segment. In ments carry a glutamate in the third transmembrane visual pigments with absorption maxima greater than segment (corresponding to glutamate 113 in bovine rho- z440 nm, this Schiff base is protonated and therefore dopsin), which serves as the counterion to the proton- Figure 2. Dendrogram of Chicken, Mouse, and Human Visual Pigments The branch lengths of the dendrogram corre- spond to percent amino acid divergence. The absorption maximum of each pigment is indi- cated by the vertical position of its branch relative to the wavelength axis (right). Review 301 Figure 3. Retinal, Polyene, and Cyanine Struc- tures, Each with Six Double Bonds, Showing p Electron Delocalization and Absorption Maxima (A) Photoisomerization of a protonated Schiff base of 11-cis retinal to all-trans retinal, as occurs in vertebrate visual pigments. (B and C) Major (upper) and minor (lower) res- onance structures of (B) a protonated Schiff base of 11-cis retinal and (C) a polyene. (D) Equivalent resonance structures of a cy- anine. ated retinylidene Schiff base (Sakmar et al., 1989; Zhu- pigmentsÐ180 in the fourth transmembrane segment kovsky and Oprian, 1989; Nathans, 1990). As illustrated and 277 and 285 in the sixth transmembrane segmentÐ in Figure 3B, the positive charge on the protonated Schiff account for most of the spectral shift between the hu- base is partially delocalized by alternate resonance struc- man red and green pigments and between the many tures. This delocalization is relevant to spectral tuning, varieties of .500 nm pigments found among New World because the spectral consequences of any perturbation primates (Neitz et al., 1991; Merbs and Nathans, 1992a, can be understood by assessing its effect on p electron 1993; Williams et al., 1992; Asenjo et al., 1994). Compari- delocalization within the 11-cis retinal chromophore: an sons of .500 nm pigment gene sequences from New increase in delocalization leads to a red shift and a and Old World primates indicate that the same dimor- decrease in delocalization leads to a blue shift (Kropf phic amino acid substitutions at these three positionsÐ and Hubbard, 1958; Mathies and Stryer, 1976; Ottolen- alanine/serine at 180, tyrosine/phenylalanine at 277, and ghi and Sheves, 1989). This effect is seen most dramati- alanine/threonine at 285Ðarose independently in multi- cally in a comparison of model compounds that repre- ple evolutionary lineages (Table 1; Shyue et al., 1995; sent the extremes of p electron delocalization (Suzuki, Hunt et al., 1998). Within the primate .500 nm cone pig- 1967). A polyene with six double bonds (Figure 3C) has ment subfamily, naturally occurring substitutions at all extreme bond length alternation, minimal p electron de- other sites produce a combined spectral shift of z5nm localization, and a peak absorption at z360 nm. By (Figure 4B; Merbs and Nathans, 1993; Asenjo et al., contrast, the corresponding cyanine (Figure 3D) has 1994; Neitz et al., 1995; Crognale et al., 1998, 1999; equivalent bond lengths, maximal p electron delocaliza- Sharpe et al., 1998). tion, and a peak absorption at z750 nm. The serine/alanine dimorphism at position 180 is of Current evidence suggests that vertebrate visual pig- special interest because it exists as a common polymor- ments modify the electronic environment of 11-cis reti- phic variant in the human gene pool, with z60% of red nal by (1) modulating the interaction between the proton- pigment genes coding for serine and z40% for alanine ated Schiff base and its counterion, and (2) modifying (Nathans et al., 1986a; Winderickx et al., 1992b; Sharpe the dipolar environment of the conjugated chromophore et al., 1998). Variation at position 180 is less common backbone with neutral amino acid side chains. For ex- among human green pigment genes, as at least 90% ample, weakening the interaction between the proton- code for alanine.